Aircraft noise is a significant issue with both economic and public health implications, for environments both internal and external to the aircraft. As a result, increasingly stringent international constraints are being placed on manufacturers to reduce this noise. Emphasis has been placed on the development of concepts to reduce this noise, whether at the source (e.g., modifications to the shape of the engine fan) or along the propagation path (e.g., installation of acoustic liners in the walls of the engine nacelle). These approaches have significantly reduced aircraft noise, but further noise reduction is needed.
Technologies used in the reduction of turbofan engine noise include, but are not limited to: (1) shield or direct noise from the engine by location and shape of the engine nacelle, (2) absorb and/or redirect noise utilizing acoustic liners mounted in the walls of an engine nacelle, (3) modify fan geometry (e.g., lean and sweep of blades, rotor and/or stator spacing) to reduce noise generated by the fans, and (4) increase bypass ratio to reduce the strength of jet noise at the source.